(1) Field of the Invention
The present invention relates mainly to an electron beam application technology adaptable to a scanning electron microscope (SEM) that uses an electron beam to permit observation of a microstructure.
(2) Description of the Related Art
A magnetic lens composed of coils and a magnetic circuit is usually adopted as an objective lens to be included in conventional scanning electron microscopes (SEM). A strong magnetic field is needed in order to focus a beam of electrons on a microscopic point, and an amount of current flowing through the coils has to be increased accordingly. Therefore, measures have to be taken against a rise in temperature caused by heat dissipated from the coils. Namely, a thick wire is wound in order to form the coils or a cooling water pipe is employed. Moreover, since an adiabatic state is established in a vacuum in a sample chamber, a vacuum sealing structure is needed. For these reasons, the objective lens included in a high-resolution scanning electron microscope (SEM) is large in size or the diameter or height of the objective lens ranges from 15 cm to 40 cm.
On the other hand, as a compact high-resolution SEM, an SEM described in, for example, Japanese Examined Patent Application Publication No. 7-1681 (FIG. 1) is known. Herein, an electrostatic lens is used to focus an electron beam. While an electrode outside an objective lens is retained at 0 V, a positive high voltage is applied to an electrode in the upper part of the objective lens in order to accelerate electrons. Thus, the high-resolution compact SEM is realized. When an objective lens is realized using the electrostatic lens, the objective lens can be so small in size that the diameter thereof is several centimeters or less. However, the upper limit of a resolution ranges from about 4 nm to about 6 nm. The higher resolution cannot be attained because of restrictions derived from a voltage the electrostatic lens can withstand and aberration the electrostatic lens brings about.
The idea of adopting a permanent magnet as a magnetic field source for a magnetic lens instead of coils has already been disclosed in 1950s. For example, “Permanent Magnet Lenses” written by J. H. Reisner (Journal of Applied Physics, Vol. 22, 1951, pp. 561) (Related Art 1) describes a structure adaptable to the magnetic field source. As shown in FIG. 2A, the structure has four bar magnets 20 disposed in parallel with one another at axially symmetrical positions. An outer magnetic path 21 and an inner magnetic path 22 that are axially symmetrical structures made of a soft magnetic material are magnetically connected to the south and north poles of the bar magnets. Electron lenses 24 take place in two respective places in each of which the magnetic path structures have a gap between them. By sophisticating this technology, a compact electron microscope permitting adjustment of a focal length has been realized. For example, “A New Permanent Magnet Lens System and its Characteristics” written by H. Kimura (Applied Physics, Vol. 26, 1957, pp. 45) (Related Art 2) describes such a compact electron microscope. Herein, two or three lenses are each composed of a permanent magnet and a magnetic circuit including soft magnetic substances.